To secure the loan, Leibovitz put up as security four properties in New York and the copyright to every picture she has taken, as well as those she will take in the future.
The lending company say she sought the loan to deal with a "dire financial condition arising from her mortgage obligations, tax liens [interest] and unpaid bills to service providers and other creditors."
But ACG said Leibovitz breached an agreement granting it the right to sell the security items before the loan was due to be repaid.
The lawsuit alleges she did not allow estate agents into her homes to value them and refused to allow ACG to sell her photographs.
Spokesman Montieth Illingworth said on Friday that ACG had "clear contractual rights and will protect them in any scenario".
ACG said Leibovitz's body of work is worth about $40m, while estate agents say that the four properties put up as security are worth about the same sum.
Illingworth said that the sales agreement with Leibovitz was intended to secure ACG 10 per cent commission on the sale of Leibovitz's real estate and 15 per cent on the sale of her portfolio.
Leibovitz would keep the remainder from the sales, less the value of the original loan, interest and fees, Illingworth said.
The photographer was accused last week by an Italian photographer in a Manhattan federal lawsuit of using his images in a calendar without his consent.
Leibovitz has been one of the world's most revered photographers for the last 40 years.
Her portraiture has featured on the covers of magazines Vanity Fair and Vogue and she has photographed John Lennon, Queen Elizabeth II and Tom Cruise.